DIP idea: q{}-inspired block mixins

[Q. Schroll]

Mixin declarations and statements are often used in conjunction 
with large literals containing lots of code, where stuff is 
inserted at specific points using interruptions like mixin("...", 
identifier, "...") or %s format specifiers and .format afterwards.
It hurts readability. In the best case, code should directly 
express what we mean.
We mean in those cases: Insert the value of identifier and not 
"identifier" here.
An observation is also that hardly anywhere, the value of the 
identifier and not "identifier" are used together.

Suggestion:

     mixin[op]
     {
         lhs op= rhs;
     }

It parses the stuff between the { and } the same as a q{}-string, 
but any identifier token found that is identical to the one 
between [ and ] will not be used literally, but be evaluated 
(mixed in). So the above is equivalent to:

     mixin("lhs ", op, "= rhs;");

For a simple idea(*): Pack everything between the braces in a q{} 
string. "Interrupt" the q{} string anywhere an identifier `ident` 
of the bracketed list is found by "},ident,q{".
(*) Doesn't work when braces are involved.

A prime example is pseudo-code we have on the spec that doesn't 
compile. We often write stuff like op= and expect people to 
interpret it properly. With that, the compiler could, too.

FAQ.

Q: Can I use multiple identifiers?
A: Yes. Comma-separated in the brackets. E.g. [ident1, ident2]

Q: Can I use expressions?
A: Instead of a plain identifier, use [ident = expression()].

Q: So [ident] alone is equivalent to [ident = ident]?
A: You got the idea right, but you cannot actually write that. 
Use [ident] or [ident2 = ident].

Q: Can I mix assignments and single identifiers?
A: Sure.

Q: Where can I use it?
A: Anywhere a mixin() statement or declaration can be. It's just 
an alternative syntax to mixin per se. Potentially also 
everywhere a {}-block can be. (Let's see what others think about 
that.)

Q: Wait, what about mixin() expressions?
A: Unfortunately not those. Block mixins are for complete 
statements or delcarations (depending on the context). Note that 
the examples didn't use a semicolon at their end.

Q: Can I use the identifiers in [ and ] in the block mixin 
without being mixed in (i.e. "escaped" in some sense)?
A: No. Use [ident2 = ident]. Then ident will not be replaced by 
its value.

Syntax up for debate.

Disclaimer: I don't really know how q{} strings are lexed and 
parsed. I'd assume the they're lexed as separate tokens, parsed 
token-wise until the closing brace is found.

What do you think about it?